The proposed work seeks to explain why the lac repressor binds so tightly and selectively to its operator site on DNA. It is already known that the structure of the DNA is altered by repressor binding. We are looking for corresponding changes in the protein's structure. Quite likely, any conformational changes within the protein-DNA complex serve to strengthen the tightness of binding. We have attached fluorescent probes to the protein in order to observe conformational changes. Some probes, e.g., l-anilino-8-naphthalene sulfonate, bind non-covalently. Others have been selectively attached to sulfhydryl groups. Currently, we are attempting to convalently attach probes to specific sites within the amino terminal region. These fluorescent probes report conformational changes which alter their local environments. We are studying the rapid kinetics of these changes with stopped-flow fluorimetric techniques. Our results indicate that the structure of the protein is influenced by binding to DNA. We are working to provide a detailed kinetic description of this process. By comparing non-specific and operator DNA, we hope to reveal the nature of the specificity for operator DNA. BIBLIOGRAPHIC REFERENCES: S.S. York, L. Yang and D. M. Worah, "l-Anilino-8-Naphthalene Sulfonate Binding to Lac Repressor: A Probe of Repressor-Poly(A-T) Association", Fed. Proc. 35, 1491 (1976).